Soheila Najafzadeh

Elevated brain cannabinoid CB1 receptor availability in post-traumatic stress disorder: a positron emission tomography study.

Endocannabinoids and their attending cannabinoid type 1 (CB1) receptor have been implicated in animal models of post-traumatic stress disorder (PTSD). However, their specific role has not been studied in people with PTSD. Herein, we present an in vivo imaging study using positron emission tomography (PET) and the CB1-selective radioligand [11C]OMAR in individuals with PTSD, and healthy controls with lifetime histories of trauma (trauma-exposed controls (TC)) and those without such histories (healthy controls (HC)). Untreated individuals with PTSD (N=25) with non-combat trauma histories, and TC (N=12) and HC (N=23) participated in a magnetic resonance imaging scan and a resting PET scan with the CB1 receptor antagonist radiotracer [11C]OMAR, which measures the volume of distribution (VT) linearly related to CB1 receptor availability. Peripheral levels of anandamide, 2-arachidonoylglycerol, oleoylethanolamide, palmitoylethanolamide and cortisol were also assessed. In the PTSD group, relative to the HC and TC groups, we found elevated brain-wide [11C]OMAR VT values (F(2,53)=7.96, P=0.001; 19.5% and 14.5% higher, respectively), which were most pronounced in women (F(1,53)=5.52, P=0.023). Anandamide concentrations were reduced in the PTSD relative to the TC (53.1% lower) and HC (58.2% lower) groups. Cortisol levels were lower in the PTSD and TC groups relative to the HC group. Three biomarkers examined collectively—OMAR VT, anandamide and cortisol—correctly classified nearly 85% of PTSD cases. These results suggest that abnormal CB1 receptor-mediated anandamide signaling is implicated in the etiology of PTSD, and provide a promising neurobiological model to develop novel, evidence-based pharmacotherapies for this disorder.

Studies of the Metabotropic Glutamate Receptor 5 Radioligand [11C]ABP688 with N-acetylcysteine Challenge in Rhesus Monkeys

Detecting changes in receptor binding at the metabotropic glutamate receptor 5 (mGluR5) with the PET allosteric antagonist, [11C]ABP688, may be valuable for studying dysfunctional glutamate transmission associated with psychiatric illnesses. This study was designed to validate the findings of a recent pilot study in baboons which reported a significant global decrease from baseline [11C]ABP688 binding after increasing endogenous glutamate with 50 mg/kg N‐acetylcysteine (NAC), with no change from test to retest. In rhesus monkeys (n = 5), paired [11C]ABP688 scans were performed on the same day on the Focus‐220 as follows (n = 3 per group): test‐retest, baseline‐NAC (50 mg/kg), and baseline‐NAC (100 mg/kg). Multiple modeling methods were evaluated for kinetic analysis to estimate the total volume of distribution (VT) and non‐displaceable binding potential (BPND) in regions‐of‐interest (ROIs), with the cerebellum gray matter (CGM) as the reference region. There was an increasing trend from test to retest BPND across ROIs (13%). NAC (50 mg/kg and 100 mg/kg) increased VT (5% and 19%) and decreased BPND (3% and 10%), respectively, significant only for VT in ROIs at the 100 mg/kg dose. High intersubject variability in BPND was comparable to that reported in the baboon study. However, interpretability of BPND is difficult with increases in VT in the CGM reference region at the higher NAC dose. Additionally, the net reduction in BPND from the baseline‐NAC scans may be obscured due to observed increases in test‐retest BPND. Thus, we did not strictly replicate the findings in the baboon study based on BPND. Synapse 67:489–501, 2013.

Synthesis and Preclinical Evaluation of 11C-UCB-J as a PET Tracer for Imaging the Synaptic Vesicle Glycoprotein 2A in the Brain

The synaptic vesicle glycoprotein 2A (SV2A) is found in secretory vesicles in neurons and endocrine cells. PET with a selective SV2A radiotracer will allow characterization of drugs that modulate SV2A (e.g., antiepileptic drugs) and potentially could be a biomarker of synaptic density (e.g., in neurodegenerative disorders). Here we describe the synthesis and characterization of the SV2A PET radiotracer 11C-UCB-J ((R)-1-((3-(11C-methyl-11C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) in nonhuman primates, including whole-body biodistribution. Methods: 11C-UCB-J was prepared by C-11C-methylation of the 3-pyridyl trifluoroborate precursor with 11C-methyl iodide via the Suzuki–Miyaura cross-coupling method. Rhesus macaques underwent multiple scans including coinjection with unlabeled UCB-J (17, 50, and 150 μg/kg) or preblocking with the antiepileptic drug levetiracetam at 10 and 30 mg/kg. Scans were acquired for 2 h with arterial sampling and metabolite analysis to measure the input function. Regional volume of distribution (VT) was estimated using the 1-tissue-compartment model. Target occupancy was assessed using the occupancy plot; the dissociation constant (Kd) was determined by fitting self-blocking occupancies to a 1-site model, and the maximum number of receptor binding sites (Bmax) values were derived from baseline VT and from the estimated Kd and the nondisplaceable distribution volume (VND). Results: 11C-UCB-J was synthesized with greater than 98% purity. 11C-UCB-J exhibited high free fraction (0.46 ± 0.02) and metabolized at a moderate rate (39% ± 5% and 24% ± 3% parent remaining at 30 and 90 min) in plasma. In the monkey brain, 11C-UCB-J displayed high uptake and fast kinetics. VT was high (∼25–55 mL/cm3) in all gray matter regions, consistent with the ubiquitous expression of SV2A. Preblocking with 10 and 30 mg/kg of levetiracetam resulted in approximately 60% and 90% occupancy, respectively. Analysis of the self-blocking scans yielded a Kd estimate of 3.4 nM and Bmax of 125–350 nM, in good agreement with the in vitro inhibition constant (Ki) of 6.3 nM and regional Bmax in humans. Whole-body biodistribution revealed that the liver and the brain are the dose-limiting organs for males and females, respectively. Conclusion: 11C-UCB-J exhibited excellent characteristics as an SV2A PET radiotracer in nonhuman primates. The radiotracer is currently undergoing first-in-human evaluation.

Reductions in Brain 5-HT1B Receptor Availability in Primarily Cocaine-Dependent Humans

BACKGROUND Preclinical evidence implicates the serotonin receptor 5-hydroxytryptamine 1B (5-HT1B) in the effects of cocaine. This study explores 5-HT1B in humans by examining receptor availability in vivo in subjects whose primary addiction is cocaine dependence (CD) using positron emission tomography. METHODS Study participants included 14 medically healthy subjects with CD (mean age = 41 ± 6 years) who were compared with 14 age-matched healthy control subjects (mean age = 41 ± 8 years) with no past or current history of cocaine or other illicit substance abuse. Participants underwent magnetic resonance imaging followed by positron emission tomography with the highly selective 5-HT1B tracer, [(11)C]P943, for purposes of quantifying regional binding potential. Voxel-based morphometry and gray matter masking also were employed to control for potential partial volume effects. RESULTS The [(11)C]P943 positron emission tomography imaging data in nine candidate regions (amygdala, anterior cingulate cortex, caudate, frontal cortex, hypothalamus, pallidum, putamen, thalamus, and ventral striatum) showed significant or nearly significant reductions of regional binding potential in subjects with CD in three regions: anterior cingulate (-16%, p < .01), hypothalamus (-16%, p = .03), and frontal cortex (-7%, p = .08). Voxel-based morphometry showed significant gray matter reductions in the frontal cortex of subjects with CD. After gray matter masking, statistically significant reductions in the [(11)C]P943 regional binding potential were either retained (anterior cingulate, -14%, p = .01; hypothalamus, -20%, p < .01) or achieved (frontal cortex, -14%, p < .01). Whole-brain voxel-wise parameter estimation confirmed these results. Secondary analyses were also significant in some regions for years of cocaine and daily tobacco use. CONCLUSIONS The reductions found in this study suggest that 5-HT1B receptors may contribute to the etiology or expression of CD and potentially represent a target for medication development.

Determination of the In Vivo Selectivity of a New κ-Opioid Receptor Antagonist PET Tracer 11C-LY2795050 in the Rhesus Monkey

11C-LY2795050 is a novel κ-selective antagonist PET tracer. The in vitro binding affinities (Ki) of LY2795050 at the κ-opioid (KOR) and μ-opioid (MOR) receptors are 0.72 and 25.8 nM, respectively. Thus, the in vitro KOR/MOR binding selectivity is about 36:1. Our goal in this study was to determine the in vivo selectivity of this new KOR antagonist tracer in the monkey. Methods: To estimate the ED50 value (dose of a compound [or drug] that gives 50% occupancy of the target receptor) of LY2795050 at the MOR and KOR sites, 2 series of blocking experiments were performed in 3 rhesus monkeys using 11C-LY2795050 and 11C-carfentanil with coinjections of various doses of unlabeled LY2795050. Kinetic modeling was applied to calculate regional binding potential (BPND), and 1- and 2-site binding curves were fitted to these data to measure 11C-LY2795050 binding selectivity. Results: The LY2795050 ED50 at MOR was 119 μg/kg based on a 1-site model for 11C-carfentanil. The 1-site binding model was also deemed sufficient to describe the specific binding of 11C-LY2795050 at KOR. The ED50 at KOR estimated from the 1-site model was 15.6 μg/kg. Thus, the ED50 ratio for MOR:KOR was 7.6. Conclusion: The in vivo selectivity of 11C-LY2795050 for KOR over MOR is 7.6. 11C-LY2795050 has 4.7-fold-lower selectivity at KOR over MOR in vivo as compared with in vitro. Nevertheless, on the basis of our finding in vivo, 88% of the PET-observed specific binding of 11C-LY2795050 under baseline conditions will be due to binding of the tracer at the KOR site in a region with similar prevalence of KOR and MOR. 11C-LY2795050 is sufficiently selective for KOR over MOR in vivo to be considered an appropriate probe for studying the KOR with PET.

Test–Retest Reproducibility of Binding Parameters in Humans with 11C-LY2795050, an Antagonist PET Radiotracer for the κ Opioid Receptor

11C-LY2795050 is a new antagonist PET radioligand for the κ opioid receptor (KOR). In this study, we assessed the reproducibility of the binding parameters of 11C-LY2795050 in healthy human subjects. Methods: Sixteen healthy subjects (11 men and 5 women) underwent 2 separate 90-min PET scans with arterial input function and plasma free fraction (fP) measurements. The 2-tissue-compartment model and multilinear analysis-1 were applied to calculate 5 outcome measures in 14 brain regions: distribution volume (VT), VT normalized by fP (VT/fP), and 3 binding potentials (nondisplaceable binding potential, binding potential relative to total plasma concentration, and binding potential relative to free plasma concentration: BPND, BPP, BPF, respectively). Since KOR is distributed ubiquitously throughout the brain, there are no suitable reference regions. We used a fixed fraction of individual cerebellar VT value (VT,CER) as the nondisplaceable VT (VND) (VND = VT,CER/1.17). The relative and absolute test–retest variability and intraclass correlation coefficient were evaluated for the outcome measures of 11C-LY2795050. Results: The test–retest variability of 11C-LY2795050 for VT was no more than 10% in any region and was 12% in the amygdala. For binding potential (BPND and BPP), the test–retest variability was good in regions of moderate and high KOR density (BPND > 0.4) and poor in regions of low density. Correction by fP (VT/fP or BPF) did not improve the test–retest performance. Conclusion: Our results suggest that quantification of 11C-LY2795050 imaging is reproducible and reliable in regions with moderate and high KOR density. Therefore, we conclude that this first antagonist radiotracer is highly useful for PET studies of KOR.

Further evaluation of [11C]MP-10 as a radiotracer for phosphodiesterase 10A: PET imaging study in rhesus monkeys and brain tissue metabolite analysis.

[11C]MP‐10 is a potent and specific PET tracer previously shown to be suitable for imaging the phosphodiesterase 10A (PDE10A) in baboons with reversible kinetics and high specific binding. However, another report indicated that [11C]MP‐10 displayed seemingly irreversible kinetics in rhesus monkeys, potentially due to the presence of a radiolabeled metabolite capable of penetrating the blood‐brain‐barrier (BBB) into the brain. This study was designed to address the discrepancies between the species by re‐evaluating [11C]MP‐10 in vivo in rhesus monkey with baseline scans to assess tissue uptake kinetics and self‐blocking scans with unlabeled MP‐10 to determine binding specificity. Ex vivo studies with one rhesus monkey and 4 Sprague‐Dawley rats were also performed to investigate the presence of radiolabeled metabolites in the brain. Our results indicated that [11C]MP‐10 displayed reversible uptake kinetics in rhesus monkeys, albeit slower than in baboons. Administration of unlabeled MP‐10 reduced the binding of [11C]MP‐10 in a dose‐dependent manner in all brain regions including the cerebellum. Consequently, the cerebellum appeared not to be a suitable reference tissue in rhesus monkeys. Regional volume of distribution (VT) was mostly reliably derived with the multilinear analysis (MA1) method. In ex vivo studies in the monkey and rats only negligible amount of radiometabolites was seen in the brain of either species. In summary, results from the present study strongly support the suitability of [11C]MP‐10 as a radiotracer for PET imaging and quantification of PDE10A in nonhuman primates. Synapse 69:86–95, 2015.

Tracer Kinetic Modeling of [11C]AFM, a New PET Imaging Agent for the Serotonin Transporter

[11C]AFM, or [11C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine, is a new positron emission tomography (PET) radioligand with high affinity and selectivity for the serotonin transporter (SERT). The purpose of this study was to determine the most appropriate kinetic model to quantify [11C]AFM binding in the healthy human brain. Positron emission tomography data and arterial input functions were acquired from 10 subjects. Compartmental modeling and the multilinear analysis-1(MA1) method were tested using the arterial input functions. The one-tissue model showed a lack of fit in low-binding regions, and the two-tissue model failed to estimate parameters reliably. Regional time–activity curves were well described by MA1. The rank order of [11C]AFM binding potential (BPND) matched well with the known regional SERT densities. For routine use of [11C]AFM, several noninvasive methods for quantification of regional binding were evaluated, including simplified reference tissue models (SRTM and SRTM2), and multilinear reference tissue models (MRTM and MRTM2). The best methods for region of interest (ROI) analysis were MA1, MRTM2, and SRTM2, with fixed population kinetic values (k′2 or b′) for the reference methods. The MA1 and MRTM2 methods were best for parametric imaging. These results showed that [11C]AFM is a suitable PET radioligand to image and quantify SERT in humans.

Assessing Synaptic Density in Alzheimer Disease With Synaptic Vesicle Glycoprotein 2A Positron Emission Tomographic Imaging

Importance Synaptic loss is well established as the major structural correlate of cognitive impairment in Alzheimer disease (AD). The ability to measure synaptic density in vivo could accelerate the development of disease-modifying treatments for AD. Synaptic vesicle glycoprotein 2A is an essential vesicle membrane protein expressed in virtually all synapses and could serve as a suitable target for synaptic density. Objective To compare hippocampal synaptic vesicle glycoprotein 2A (SV2A) binding in participants with AD and cognitively normal participants using positron emission tomographic (PET) imaging. Design, Setting, and Participants This cross-sectional study recruited 10 participants with AD and 11 participants who were cognitively normal between November 2015 and June 2017. We hypothesized a reduction in hippocampal SV2A binding in AD, based on the early degeneration of entorhinal cortical cell projections to the hippocampus (via the perforant path) and hippocampal SV2A reductions that had been observed in postmortem studies. Participants underwent high-resolution PET scanning with ((R)-1-((3-(11C-methyl-11C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one), a compound more commonly known as 11C-UCB-J, for SV2A. They also underwent high-resolution PET scanning with carbon 11–labeled Pittsburgh Compound B (11C-PiB) for &bgr;-amyloid, magnetic resonance imaging, and cognitive and neurologic evaluation. Main Outcomes and Measures Outcomes were 11C-UCB-J–specific binding (binding potential [BPND]) via PET imaging in brain regions of interest in participants with AD and participants who were cognitively normal. Results Ten participants with AD (5 male and 5 female; mean [SD] age, 72.7 [6.3] years; 10 [100%] &bgr;-amyloid positive) were compared with 11 participants who were cognitively normal (5 male and 6 female; mean [SD] age, 72.9 [8.7] years; 11 [100%] &bgr;-amyloid negative). Participants with AD spanned the disease stages from amnestic mild cognitive impairment (n = 5) to mild dementia (n = 5). Participants with AD had significant reduction in hippocampal SV2A specific binding (41%) compared with cognitively normal participants, as assessed by 11C-UCB-J–PET BPND (cognitively normal participants: mean [SD] BPND, 1.47 [0.37]; participants with AD: 0.87 [0.50]; P = .005). These reductions remained significant after correction for atrophy (ie, partial volume correction; participants who were cognitively normal: mean [SD], 2.71 [0.46]; participants with AD: 2.15 [0.55]; P = .02). Hippocampal SV2A-specific binding BPND was correlated with a composite episodic memory score in the overall sample (R = 0.56; P = .01). Conclusions and Relevance To our knowledge, this is the first study to investigate synaptic density in vivo in AD using 11C-UCB-J–PET imaging. This approach may provide a direct measure of synaptic density, and it therefore holds promise as an in vivo biomarker for AD and as an outcome measure for trials of disease-modifying therapies, particularly those targeted at the preservation and restoration of synapses.

Imaging the cannabinoid CB1 receptor in humans with [11C]OMAR: Test–retest reproducibility and gender differences

Methods: Eight healthy volunteers (4 males and 4 females) each underwent two [C]OMAR scans on the same day with injections separated by ∼3.5 h; 2 additional subjects (1 male and 1 female) were scanned once apiece. PET data were collected for 120 min on the HRRT scanner and arterial input functions were measured. Injected activity was 18±1 mCi, specific activity was 4.7±1.5 mCi/nmol, and mass dose was 0.03± 0.01 μg/kg. Distribution volume (VT) was estimated from regional time–activity curves (TACs) using a 2-tissue model (2T) with vascular component and multilinear analysis (MA1). Within-subject changes in VT between scans 1 and 2 were calculated: ΔVT=2⁎(VT1−VT2)/(VT1+ VT2)⁎100%. Plasma metabolite fraction, net tracer uptake (SUV), and VT were assessed for gender differences using two-tailed t-tests (uncorrected) with significance at p<0.05.